Nanocomposite soft magnetic materials comprise crystalline grains smaller than 100 nm embedded within an amorphous matrix. Extensive research into this class of materials resulted in their use in a wide range of soft magnetic applications. The extended set of processing dimensions in nanocomposites, such as grain size, phase identity, crystal orientation and volume fraction, present opportunities to tailor properties. Several classes of nanocomposite alloys exist, e.g., as described in U.S. Pat. No. 4,881,989, U.S. Pat. No. 5,935,347, and U.S. Patent Publication No. 2010/0265028. Compositions typically consist of compositions (TL-TE-M) involving combinations of late transition metals (TL), early transition metals (TE), and metalloids (M). Compositions (such as those described in U.S. Pat. No. 5,935,347) also require small additions of noble metals such as Cu to promote finer grain structures. The Curie temperature of the amorphous phase in iron (Fe) rich nanocomposite alloys is typically below 400° C. and soft magnetic properties degrade approaching and above this temperature. Cobalt additions can increase the amorphous phase Curie temperature, as described in U.S. Patent Publication No. 2010/0265028, and extend useful range of operating temperatures.
The introduction of a uniaxial magnetic anisotropy in nanocomposite soft magnetic materials is a well-known technique to control permeability. Magnetic permeability (μ) is the change in induction (B) with an applied field (H) and is equal to the slope of a magnetization curve. Both high and low permeabilities may be advantageous depending on the application and it is often desirable to tune permeability to a specific value. In particular, flat loop magnetization with linear response of the magnetization with applied field, to a field designated as the anisotropy field, HK, where the magnetization reaches it saturation value, Ms, are desirable for many applications. In flat loop materials the permeability at a fixed temperature remains approximately single valued up to the saturating field, HK, and given by:μ=Ms\HK 
For low permeability materials in toroidal cores that show linear magnetization responses, the anisotropy energy density Ku is:
      K    u    =                              M          s                ⁢                  H          k                    2        =                            B          s          2                          2          ⁢          μ                    .      
Here, Bs is the saturation induction and the permeability μ is inversely proportional to the energy density for wound cores with transverse induced anisotropy.
The manufacturing process to make nanocomposites of this type typically include an annealing step where one or more crystalline phases is nucleated and grown within an amorphous precursor material. Several processing methods are available during annealing to develop magnetic anisotropies including the introduction of a magnetic and/or stress field. The use of an amorphous precursor limits the geometry of nanocomposite soft magnets to a critical cooling dimension and these materials often take the shape of cast ribbons resulting from a melt spinning technique or wires drawn from a melt. These ribbons may be tape wound or stamped and stacked (as described in U.S. Patent Publication No. 2001/0043134) into cores for use. Stresses in the final component include residual stresses formed during the casting process, those induced during the annealing, as well as any further processing step such as winding, impregnating, and cutting. Successful devices require adequate control over all processing steps to include or reduce stresses where desired for a specific application.
The induced magnetic anisotropy and demagnetization field, Hd, created by the core shape influences the domain pattern in the core. Free magnetic poles on the surface create the demagnetization field, which tend to bend field lines in the material. Toroidal shapes minimize demagnetization effects for materials with domains oriented along the circumference. Referring to FIG. 1, diagram 100 shows two domain orientations in a tape wound core, e.g., domain patterns in strain annealed tape-wound cores with transverse (left) patterns 102 and longitudinal (right) domain patterns 104. The direction of the induced anisotropy and the demagnetization field of the core shape determine the primary domain orientation. Annealing materials with negative and positive magnetostriction (λ) in the crystalline phase under tension can result in transverse and longitudinal domain patterns respectively depending on annealing conditions. Dynamic magnetization processes are complicated but transverse domains in wound core material tend to change magnetization by rotation whereas longitudinal domains change magnetization by domain wall motion. The sign of the magnetostriction in the crystalline phase of Fe—Si nanocomposites controls the direction of the induced easy axis within a plane oriented based on the primary stress direction.
The direction of the magnetic easy axis determines permeability of the core. Referring to FIG. 2, diagram 110 shows the resulting magnetization vs. field curves for soft magnetic cores with different induced magnetic anisotropies. In particular, diagram 110 shows magnetization vs. field curves of A) annealed core without an applied field/stress, B) longitudinally field annealed or strain annealed core resulting in primarily longitudinal domains, and C) transverse field annealed or strain annealed core resulting in primarily transverse domains. Curve A results after annealing without an external magnetic/stress field and is the result of the well know random exchange phenomenon. The high permeability shown in Curve B can result from annealing in a longitudinal magnetic field or by strain annealing ribbon with positive magnetostriction in the crystalline phase under tension. The longitudinal magnetic field is applied in the same direction as the excitation field created by the windings on a toroidal core (circumferential). The low permeability shown in Curve C can result from annealing cores in a transverse magnetic field (perpendicular to the excitation field) and/or strain annealing ribbon with negative magnetostriction in the crystalline phase.
Properties in nanocomposites may differ from as-cast or mostly amorphous materials of similar compositions. Magnetostriction is primarily determined by composition and structure of the material, temperature, and stress. The crystalline phase composition often differs from the residual amorphous phase composition and apparent magnetostriction values are composed of contributions from each phase. Crystalline phases also offer additional processing variables such as ordering of atoms/defects, preferred orientations, and the volume fraction of crystals that are useful in the development of anisotropies. In strain annealed Fe-rich nanocomposite cores, the resulting permeability shows an inverse relationship to the magnitude of the stress field applied during annealing. Determination of the underlying mechanism behind the permeability change in magnetic field annealing has proven elusive, but evidence of residual lattice strain in strain annealed materials has been shown. (See, e.g., Ohnuma et al., “Origin of the magnetic anisotropy induced by stress annealing in Fe-based nanocrystalline alloy,” Appl. Phys. Lett., vol. 86, no. 152513, pp. 1-4, 2005). This has been duplicated and the effect of strain annealing materials with positive and negative crystal phase magnetostrictions in the FeSi composition range has been demonstrated. (See, e.g., Ershov et al., “Relaxation of the state with induced transverse magnetic anisotropy in the soft magnetic nanocrystalline alloy Fe73.5Si13.5Nb3B9Cu1,” Physics of the Solid State, vol. 54, no. 9, pp. 1817-1826, September 2012. http://www.springerlink.com/index/10.1134/S1063783412090119; and Ershov et al., “Effect of thermomagnetic and thermomechanical treatments on the magnetic properties and structure of the nanocrystalline soft magnetic alloy Fe81Si6Nb3B9Cu1,” Physics of the Solid State, vol. 55, no. 3, pp. 508-519, March 2013. http://link.springer.com/10.1134/S1063783413030098). This work also showed that while the anisotropy induced by magnetic field annealing is largely erasable at temperatures above ˜250° C., strain induced anisotropies were stable to much higher temperatures. However, the low Curie temperature of the amorphous phase in Fe-rich alloys prevents the exploitation of the thermal stability of the induced anisotropy for elevated temperature applications. Additionally, the brittle nature of crystallized Fe-rich compositions limits the accessible range of induced anisotropies and make core winding difficult.
Nanocomposite ribbons can be subsequently crushed into flake and then consolidated by compaction techniques. The uniaxial anisotropy can be exploited to align the flakes in an applied field prior to compaction. This alloys for retaining anisotropy in shapes possible through consolidation.
Fluxgate and giant magnetoimpedance (GMI) magnetic field sensors require soft magnetic sensing elements. The sensing element in a fluxgate magnetometer is typically a soft magnetic core that is alternately driven to saturation by a drive coil. (See D. I. Gordon and R. E. Brown, “Recent Advances in Fluxgate Magnetometry,” IEEE Transactions on Magnetics, vol. MAG-8, no. 1, pp. 76-82, 1972). When placed in an external field, the core saturates asymmetrically producing even harmonics proportional to the field strength according to Faraday's law. GMI sensors measure changes in the impedance (Z=R+jωL) of a soft magnetic sensing element where R is the dc resistance and L is the inductance. (See Mohri et al., “Magneto-Impedance Element,” IEEE Transactions on Magnetics, vol. 31, no. 4, pp. 2455-2460, 1995). The skin depth of the sensing element is sensitive to permeability and resistivity. In GMI sensors, the primary mechanism that determines field sensitivity ΔZ(H)/Z(H) involves control of permeability in the sensing element. The defining measure of sensor performance is the signal to noise ratio. Random jumps in magnetization in the sensing element (Barkhausen noise) degrade performance and should be minimized through the use of a low loss sensing element. Amorphous materials have been used with success in fluxgate cores and GMI sensors, but show thermal instabilities at high temperature and are not applicable for high temperature applications such as deep drilling operations or current sensing in high temperature electronics.
Deep well drilling requires robust sensor packages that can operate in a harsh environment of corrosive fluids and temperatures and pressures in excess of 200° C. and 20 ksi. The sensor package relays parameters such as torque, temperature, and bit azimuth to an operator in real time to ensure drilling accuracy. The magnetometer in the sensor package can be used to provide azimuth information with respect to the earth's field or to home in on a separate magnetic source, as required for relief well intersections. However, the crystallization temperature of amorphous materials limits their structural stability and prevents their use in high temperature applications.
Controlling the permeability of the sensing element can be used to reduce the noise in fluxgates (See Ripka et al., “Nanocrystalline Fluxgate Cores with Transverse Anisotropy,” in Sensors, Proceedings of the IEEE, 2004, pp. 1570-1572) and to tune the field response of a GMI sensor, as described in U.S. Pat. No. 5,994,899, U.S. Pat. No. 6,747,449, US Publication No. 2004/0403725, and U.S. Pat. No. 6,727,692. Transverse anisotropy is preferable in ring or racetrack fluxgate cores and the preferred anisotropy direction in GMI sensing elements is shown in diagram 120 of FIG. 3. Fe-based nanocomposites are limited due to relatively low Curie temperatures in the amorphous phase and a limited response to stress annealing. Additionally, for materials that require robust mechanical properties, the brittle nature of Fe-based materials after crystallization limits their use. The composition of Co-rich nanocomposites can be varied to achieve brittle or ductile ribbons after crystallization, depending on the application requirement. What is needed is a soft magnetic material that is stable at high temperatures with a strong response to stress annealing. This induced anisotropy and resulting anisotropy field, in this material should also be tunable for a range of applications up a high value. Preferably, the material should exhibit adequate mechanical properties such as ductility after thermo-mechanical processing to allow for successful fabrication of robust devices.